Pressure vessels, such as nuclear pressure vessels, include manways, inspection ports, handholes and other openings to allow for inspection, maintenance and repair. During normal operation these openings must not leak, and so are covered and sealed shut by various means.
Covered openings in pressure vessels are typically circular and have a joint, between the opening and the cover, sealed by a gasket. An array of bolts or studs, arranged about the cover, provide the mechanical force needed to compress the gasket and seal the joint. For openings subject to high temperature and pressure in critical applications where leakage cannot be tolerated (e.g. nuclear steam generators), a diaphragm is widely used as a key element of the gasket sealing structure. A diaphragm is a thin, flexible dish-shaped component placed between parallel parts of a structural steel member.
As shown in FIG. 1, a known diaphragm 10 consists of a relatively thin, flexible center section 20 with a thicker section 30 around the circumference that incorporates a gasket groove 38 with a serrated spiral finish. During operation a gasket is disposed in gasket groove 38 adjacent a vessel opening. A sealing pressure is applied to the gasket using bolts or other means. With this known diaphragm 10, the gasket sealing pressure decreases as higher fluid pressure is applied to a vessel opening, and is transmitted to flexible center section 20 of diaphragm 10 via fluid-filled diaphragm opening 15. The decrease of sealing pressure on the inside diameter of the gasket circumference is much larger than on the outside diameter. Therefore, the minimum sealing pressure always occurs on the inside diameter of the gasket and serves to worsen gasket sealing performance. During hydrostatic testing or normal, steady state operation at high temperatures and pressure, known diaphragm 10 is thus not always able to maintain adequate gasket sealing pressure for these vessel openings.